Code Division Multiple Access (CDMA) is a modulation and multiple access scheme used for various communication applications, such as wireless personal communication systems and digital cellular telephony. For example, a CDMA cellular telephone system uses spread spectrum communication technology to separate one user's signal from another. Thus, as a user's or subscriber's signal is transmitted from a cellular system base station, the signals are distinguished at the subscriber unit by using a binary orthogonal spreading code, which may be based on Walsh functions (also known as Hadamard matrices).
In an IS-95 type cellular communications system (i.e., a communications system implemented according to Telecommunications Industry Association (TIA) Interim Standard (IS) 95, or the like), the Walsh function is 64 PN (pseudonoise) code chips long and represents 64 different orthogonal spreading codes. Even though other systems may use alternative spreading codes, the direct sequence spread spectrum communication system must agree upon a finite number of spreading codes that will be used to spread a signal at the base station and despread a signal at the subscriber unit.
In some instances, the number of spreading codes available is insufficient to handle the number of subscribers requesting a traffic channel from a base station. If the set of spreading codes at the base station is considered a resource that is allocated to subscribers that request a traffic channel, then a base station that runs low, or runs out of spreading codes, may be said to be resource limited because the number of available spreading codes has dropped below a predetermined threshold. When the base station runs out of spreading codes, a blocking situation exists wherein users requesting a traffic channel from the base station are denied access because spreading codes are not available.
Just as spreading codes are a resource at the base station, transmitter power is another base station resource that must be efficiently managed or allocated between users. Typically, base stations include a maximum power rating for the radio frequency power amplifier that amplifies the signals coupled to the base station antenna. Therefore, the total transmit power available at the base station must be efficiently divided among the various communication links between the base station and the subscribers receiving signals from the base station.
When power requested by subscriber units in a cell or sector approaches the maximum power available, the base station may be said to be power limited, in that the resource of transmit power is nearly exhausted. As a consequence of being power limited, the base station may enter a blocking situation wherein users desiring service through a traffic channel are denied access because the transmit power for supporting the channel is not available.
When cellular subscribers are blocked as described above, both the blocked subscriber and the cellular system operator are disappointed. The subscriber is disappointed because a call or data connection cannot be established or maintained, and the cellular system operator is disappointed because a subscriber is unhappy and the operator is denied the revenue that can be charged while the subscriber uses the system. Because of the problems caused by blocking when the resources in a base station are not properly managed, it should be apparent that a need exists for an improved method and system for managing base station resources in a code division multiple access cellular communication system.